1. Field of the Invention
This invention relates to a method for purifying air by removing gaseous formaldehyde which is present as an undesirable pollutant or contamination therein.
2. Description of the Prior Art
Formaldehyde-based products are widely used in the fields of coatings, binders and adhesives. Decorative laminates, plywood, particle board, textile finishes, paper coatings, bonded nonwoven cellulosic webs including wet-strength paper and disposable diapers, brake linings and foam insulation are among the numerous products which utilize crosslinkable resins prepared by the reaction of formaldehyde with either phenols, urea or melamine, including low molecular weight linear and cyclic analogs and prepolymers thereof.
In some cases, free formaldehyde is retained in fully cured composites, laminates and coatings. Further, gaseous formaldehyde may be released slowly from cured compositions at room temperature or at slightly elevated temperatures in the presence of water or under high humidity conditions.
Since formaldehyde is volatile and remains a gas under ambient temperature and pressure, it escapes into the air during the manufacture of the resins and the products prepared therefrom. The release of gaseous formaldehyde may also occur when the finished product, e.g. plywood, wall panelling, floorboard, foam insulation, durable press fabric, etc., is in place or in use.
Exposure to gaseous formaldehyde may result in respiratory difficulties, eye and skin irritation, headaches, vomiting and severe irritation to the mucous membranes. There is also a possibility that formaldehyde is a carcinogen.
The removal of gaseous formaldehyde from the air in facilities manufacturing or using formaldehyde-based resins has been a matter of concern for some time. The National Institute for Occupational Safety and Health (NIOSH) Method P&CAM 125, issued in 1973, describes a method for the analysis of "Formaldehyde in Air." In more recent years, the U.S. Consumer Product Safety Commission has become concerned about formaldehyde evolution from urea-formaldehyde foam in insulated buildings and from plywood and particle board in mobile homes.
In order to minimize or decrease the exposure to gaseous formaldehyde, various methods have been used to remove the formaldehyde from the air. Thus, the air has been passed through scrubbing towers or filters containing liquid absorbents, e.g. water or an aqueous solution or ammonium bicarbonate, or solid absorbents, e.g. activated carbon, resulting in absorption of the gaseous formaldehyde in the atmosphere. Oxidation of the formaldehyde has been promoted by passing the air through or over alkaline earth metal peroxides or alumina or clay pellets impregnated with potassium permanganate.
Although water and liquid absorbents in scrubbing towers may be effective in manufacturing facilities where large volumes of air are recirculated, they are expensive and impractical for use in dwellings. Solid absorbents such as activated carbon are effective at normal temperatures but lose their efficiency when the temperature rises. In hot, humid climates, during the summer, when the formaldehyde vapors are particularly irritating and troublesome, the absorbing capacity of the activated carbon is not only reduced, but formaldehyde which was absorbed when the temperature and humidity were lower, e.g. during the cooler evening and night hours, is desorbed and liberated into the air during the heat and humidity of the day.
While oxidizing agents such as metal peroxides and potassium permanganate are effective in converting formaldehyde into formic acid, they are non-selective and rapidly lose their reactivity by indiscriminate oxidation of other contaminants and pollutants which have reducing characteristics, such as mercaptans, hydrogen sulfide and carbon monoxide.